In a comment on the Power.org blog, Pieter Van den Abeele wrote that Genesi will be releasing the Open Server Workstation, which will be based on the PowerPC 970 (G5) chip. The system is scheduled for release this month give or take a few weeks, and will cost $1599. Developers can also register for a free workstation.

"the AMD64 architecture can address up to 256 tebibytes of virtual address space (2^48 bytes)."

"Current implementations of the AMD64 architecture can address up to 1 tebibyte of RAM (2^40 bytes)" Less than 2^48, obviously, and much less than 2^64.

IIRC, the PPC64 too does not provide a complete 2^64 memory space. It's too early for most of us to begin worrying about the upper limits of current implementations of 64-bitness. Unless you're into supercomputing amounts of data.